Pentachlorophenol (PCP), a widely used wood preservative, may contain unwanted toxic contaminants such as chlorinated dibenzo-p-dioxin and dibenzofurans. The results of a recent investigation into the cause of a toxic syndrome in dairy animals suggests that dioxins and dibenzofurans (and in turn, PCP) may have played a role. The immunosuppression induced by 2,3,7,8-TCDD and 2,3,7,8,-TCDF in laboratory animals suggested that immune alterations may be induced in these cattle. The cattle in this study consisted of 15 (12-15 months old) female Holstein cattle. The compound was administered in the feed to achieve the desired dose. The cattle were randomly subdivided into 5 groups of 3 each and given the following doses for 120 days - Group A: Maximum tolerated dose of pure PCP; Group B: Same dose of commercial PCP; Group C: 1/3 dose B and 2/3 of pure PCP; Group D: 1/3 dose C and 9 times equal amount of pure PCP; Group E: Nontreated controls.